Debris in the coolant which collects or is trapped in fuel rod spacer grids is believed responsible for as many as 30% of known fuel rod failures. Laboratory and in-reactor experience indicate that fuel rod cladding failures can be caused by debris trapped in a grid region which reacts against the fuel rod cladding in a vibratory fashion resulting in rapid wear of the cladding. Devices which will trap a large percentage of debris before it gets to the action area have been designed.
Attempts to treat the debris problem have involved grids in the region of the lower end fitting and the lower rod support grid. One such attempt is the subject of U.S. patent application Ser. No. 020,816 entitled "Debris Catching Strainer Grid", filed Mar. 2, 1987 by Andrew J. Anthony now U.S. Pat. No 4,781,884, and assigned to the assignee of the instant invention. This grid is typically welded to the upper side of the lower end fitting. Other examples of debris strainers, or traps and grids are seen in U.S. Pat. Nos.: Reissue 27,950, 4,684,495; 4,684,496; 4,652,425; 4,678,627, 4,427,624; 4,096,032; and Japanese Application 53-8277, 1-30-1978 (Kokai 54-102493, 8-11-1979); German Auslegeschrift 1,211,342 (Anmeldetag Feb. 2, 1960); and British Patent 1,214,998. None of these attempts have been totally effective for the purpose intended for reasons of compromise between costs, pressure drop during coolant flow and debris retainability.
With increased demand for performance of nuclear fuel, the need to increase flow mixing and reduce debris induced fuel rod fretting failures has given rise to higher flow resistance fuel designs. the C-E GUARDIAN.TM. Spacer Grid, which is the subject of U.S. patent application Ser. No. 07/284,018, filed Dec. 14, 1988 by william J. Bryan, now U.S. Pat. No. 4,919,883 and assigned to the same assignee as the instant invention, increases fuel assembly pressure drop by 4%. The GUARDIAN grid substantially eliminates debris induced failures by screening flow and trapping debris which causes fuel rod fretting failures. A proposed mixing grid design, which is now under development, will add an additional 15% fuel assembly pressure drop increase. These increases in pressure drop result in core flow penalties which result from the mismatched pressure drop of a mixed fuel assembly designed core. Approximately one-third (166 ) of a core of fuel is transitioned out of the reactor during each refueling. To reduce the pressure increase, an improved hydraulic design lower end fitting has been developed and is the subject of this application.